1. Field of the Invention
The present invention relates to a hydroxide-ion-conductive dense membrane and a composite material.
2. Description of the Related Art
Layered double hydroxides (hereinafter also referred to as LDHs), such as hydrotalcite, are compounds that contain exchangeable anions between hydroxide layers. Because of their characteristics. LDHs have been used as catalysts and absorbents, as well as dispersants in polymers in order to improve the thermal resistance of the polymers. In particular, LDHs have recently been attracting attention as materials exhibiting hydroxide ion conductivity, and also studied for use as electrolytes in alkaline fuel cells or additives in catalytic layers of zinc-air batteries.
Traditional applications of LDHs (e.g., catalysts) require high specific surface area, and thus it was sufficient to synthesize and use powdery LDH for such applications. In contrast, in consideration of applications of LDHs to electrolytes in, for example, alkaline fuel cells making use of the hydrogen ion conductivity of the LDHs, a highly-densified LDH membrane is desired in order to prevent fuel gas from mixing and to achieve sufficient electromotive force.
Patent Documents 1 and 2 and Non-Patent Document 1 disclose oriented LDH membranes. These oriented LDH membranes are produced by horizontally suspending the surface of a polymer substrate in a solution containing urea and a metal salt to cause nucleation and oriented growth of LDH. The oriented LDH membranes of these Documents each show a strong peak of (003) plane in the X-ray diffraction pattern.
Zinc secondary batteries, such as nickel-zinc secondary batteries and zinc-air secondary batteries, have been developed and studied over many years. Unfortunately, these batteries have not yet been put into practice. This is due to a problem that zinc contained in the negative electrode forms dendritic crystals, i.e. dendrites, during a charge mode of the battery and the dendrites break the separator to cause short circuit between the negative electrode and the positive electrode. Thus, a strong demand has arisen for a technique for preventing the short circuit resulting from dendritic zinc in zinc secondary batteries, such as nickel-zinc secondary batteries and zinc-air secondary batteries.